<s>
Fiber-optic	B-Application
communication	I-Application
is	O
a	O
method	O
of	O
transmitting	O
information	O
from	O
one	O
place	O
to	O
another	O
by	O
sending	O
pulses	O
of	O
infrared	O
light	O
through	O
an	O
optical	B-Architecture
fiber	I-Architecture
.	O
</s>
<s>
The	O
light	O
is	O
a	O
form	O
of	O
carrier	O
wave	O
that	O
is	O
modulated	B-Application
to	O
carry	O
information	O
.	O
</s>
<s>
This	O
type	O
of	O
communication	O
can	O
transmit	O
voice	O
,	O
video	O
,	O
and	O
telemetry	O
through	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
or	O
across	O
long	O
distances	O
.	O
</s>
<s>
Optical	B-Architecture
fiber	I-Architecture
is	O
used	O
by	O
many	O
telecommunications	O
companies	O
to	O
transmit	O
telephone	O
signals	O
,	O
internet	O
communication	O
,	O
and	O
cable	O
television	O
signals	O
.	O
</s>
<s>
Researchers	O
at	O
Bell	O
Labs	O
have	O
reached	O
a	O
record	O
bandwidth	O
–	O
distance	O
product	O
of	O
over	O
kilometers	O
per	O
second	O
using	O
fiber-optic	B-Application
communication	I-Application
.	O
</s>
<s>
First	O
developed	O
in	O
the	O
1970s	O
,	O
fiber-optics	B-Architecture
have	O
revolutionized	O
the	O
telecommunications	O
industry	O
and	O
have	O
played	O
a	O
major	O
role	O
in	O
the	O
advent	O
of	O
the	O
Information	O
Age	O
.	O
</s>
<s>
Because	O
of	O
its	O
advantages	O
over	O
electrical	O
transmission	O
,	O
optical	B-Architecture
fibers	I-Architecture
have	O
largely	O
replaced	O
copper	O
wire	O
communications	O
in	O
backbone	B-General_Concept
networks	I-General_Concept
in	O
the	O
developed	O
world	O
.	O
</s>
<s>
The	O
process	O
of	O
communicating	O
using	O
fiber-optics	B-Architecture
involves	O
the	O
following	O
basic	O
steps	O
:	O
</s>
<s>
Optical	B-Architecture
fiber	I-Architecture
is	O
used	O
by	O
telecommunications	O
companies	O
to	O
transmit	O
telephone	O
signals	O
,	O
Internet	O
communication	O
and	O
cable	O
television	O
signals	O
.	O
</s>
<s>
Due	O
to	O
lower	O
attenuation	O
and	O
interference	O
,	O
optical	B-Architecture
fiber	I-Architecture
has	O
advantages	O
over	O
copper	O
wire	O
in	O
long-distance	O
,	O
high-bandwidth	O
applications	O
.	O
</s>
<s>
However	O
,	O
infrastructure	O
development	O
within	O
cities	O
is	O
relatively	O
difficult	O
and	O
time-consuming	O
,	O
and	O
fiber-optic	B-Architecture
systems	O
can	O
be	O
complex	O
and	O
expensive	O
to	O
install	O
and	O
operate	O
.	O
</s>
<s>
Due	O
to	O
these	O
difficulties	O
,	O
early	O
fiber-optic	B-Application
communication	I-Application
systems	O
were	O
primarily	O
installed	O
in	O
long-distance	O
applications	O
,	O
where	O
they	O
can	O
be	O
used	O
to	O
their	O
full	O
transmission	O
capacity	O
,	O
offsetting	O
the	O
increased	O
cost	O
.	O
</s>
<s>
The	O
prices	O
of	O
fiber-optic	B-Application
communications	I-Application
have	O
dropped	O
considerably	O
since	O
2000	O
.	O
</s>
<s>
As	O
of	O
2020	O
,	O
over	O
5	O
billion	O
kilometers	O
of	O
fiber-optic	B-Architecture
cable	I-Architecture
has	O
been	O
deployed	O
around	O
the	O
globe	O
.	O
</s>
<s>
In	O
1880	O
Alexander	O
Graham	O
Bell	O
and	O
his	O
assistant	O
Charles	O
Sumner	O
Tainter	O
created	O
a	O
very	O
early	O
precursor	O
to	O
fiber-optic	B-Application
communications	I-Application
,	O
the	O
Photophone	O
,	O
at	O
Bell	O
's	O
newly	O
established	O
Volta	O
Laboratory	O
in	O
Washington	O
,	O
D.C.	O
Bell	O
considered	O
it	O
his	O
most	O
important	O
invention	O
.	O
</s>
<s>
Due	O
to	O
its	O
use	O
of	O
an	O
atmospheric	O
transmission	O
medium	O
,	O
the	O
Photophone	O
would	O
not	O
prove	O
practical	O
until	O
advances	O
in	O
laser	O
and	O
optical	B-Architecture
fiber	I-Architecture
technologies	O
permitted	O
the	O
secure	O
transport	O
of	O
light	O
.	O
</s>
<s>
Jun-ichi	O
Nishizawa	O
,	O
a	O
Japanese	O
scientist	O
at	O
Tohoku	O
University	O
,	O
proposed	O
the	O
use	O
of	O
optical	B-Architecture
fibers	I-Architecture
for	O
communications	O
in	O
1963	O
.	O
</s>
<s>
Nishizawa	O
invented	O
the	O
PIN	O
diode	O
and	O
the	O
static	O
induction	O
transistor	O
,	O
both	O
of	O
which	O
contributed	O
to	O
the	O
development	O
of	O
optical	B-Application
fiber	I-Application
communications	I-Application
.	O
</s>
<s>
Optical	B-Architecture
fiber	I-Architecture
with	O
attenuation	O
low	O
enough	O
for	O
communication	O
purposes	O
(	O
about	O
20dB/km	O
)	O
was	O
developed	O
in	O
1970	O
by	O
Corning	O
Glass	O
Works	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
GaAs	O
semiconductor	O
lasers	O
were	O
developed	O
that	O
were	O
compact	O
and	O
therefore	O
suitable	O
for	O
transmitting	O
light	O
through	O
fiber	B-Architecture
optic	I-Architecture
cables	O
for	O
long	O
distances	O
.	O
</s>
<s>
Developed	O
for	O
Army	O
Missile	O
Command	O
in	O
Huntsville	O
,	O
Alabama	O
,	O
the	O
system	O
was	O
intended	O
to	O
allow	O
a	O
short-range	O
missile	O
with	O
video	O
processing	O
to	O
communicate	O
by	O
laser	O
to	O
the	O
ground	O
by	O
means	O
of	O
a	O
five-kilometer	O
long	O
optical	B-Architecture
fiber	I-Architecture
that	O
unspooled	O
from	O
the	O
missile	O
as	O
it	O
flew	O
.	O
</s>
<s>
After	O
a	O
period	O
of	O
research	O
starting	O
from	O
1975	O
,	O
the	O
first	O
commercial	O
fiber-optic	B-Architecture
telecommunications	O
system	O
was	O
developed	O
which	O
operated	O
at	O
a	O
wavelength	O
around	O
0.8μm	O
and	O
used	O
GaAs	O
semiconductor	O
lasers	O
.	O
</s>
<s>
Soon	O
on	O
22	O
April	O
1977	O
,	O
General	O
Telephone	O
and	O
Electronics	O
sent	O
the	O
first	O
live	O
telephone	O
traffic	O
through	O
fiber	B-Architecture
optics	I-Architecture
at	O
a	O
6Mbit/s	O
throughput	O
in	O
Long	O
Beach	O
,	O
California	O
.	O
</s>
<s>
In	O
October	O
1973	O
,	O
Corning	O
Glass	O
signed	O
a	O
development	O
contract	O
with	O
CSELT	O
and	O
Pirelli	O
aimed	O
to	O
test	O
fiber	B-Architecture
optics	I-Architecture
in	O
an	O
urban	O
environment	O
:	O
in	O
September	O
1977	O
,	O
the	O
second	O
cable	O
in	O
this	O
test	O
series	O
,	O
named	O
COS-2	O
,	O
was	O
experimentally	O
deployed	O
in	O
two	O
lines	O
(	O
9km	O
)	O
in	O
Turin	O
,	O
for	O
the	O
first	O
time	O
in	O
a	O
big	O
city	O
,	O
at	O
a	O
speed	O
of	O
140Mbit/s	O
.	O
</s>
<s>
The	O
second	O
generation	O
of	O
fiber-optic	B-Application
communication	I-Application
was	O
developed	O
for	O
commercial	O
use	O
in	O
the	O
early	O
1980s	O
,	O
operated	O
at	O
1.3μm	O
and	O
used	O
InGaAsP	O
semiconductor	O
lasers	O
.	O
</s>
<s>
These	O
early	O
systems	O
were	O
initially	O
limited	O
by	O
multi-mode	B-Device
fiber	I-Device
dispersion	O
,	O
and	O
in	O
1981	O
the	O
single-mode	B-Device
fiber	I-Device
was	O
revealed	O
to	O
greatly	O
improve	O
system	O
performance	O
,	O
however	O
practical	O
connectors	O
capable	O
of	O
working	O
with	O
single	B-Device
mode	I-Device
fiber	I-Device
proved	O
difficult	O
to	O
develop	O
.	O
</s>
<s>
Canadian	O
service	O
provider	O
SaskTel	O
had	O
completed	O
construction	O
of	O
what	O
was	O
then	O
the	O
world	O
's	O
longest	O
commercial	O
fiber	B-Architecture
optic	I-Architecture
network	O
,	O
which	O
covered	O
and	O
linked	O
52	O
communities	O
.	O
</s>
<s>
The	O
first	O
transatlantic	O
telephone	O
cable	O
to	O
use	O
optical	B-Architecture
fiber	I-Architecture
was	O
TAT-8	O
,	O
based	O
on	O
Desurvire	O
optimized	O
laser	O
amplification	O
technology	O
.	O
</s>
<s>
Third-generation	O
fiber-optic	B-Architecture
systems	O
operated	O
at	O
1.55μm	O
and	O
had	O
losses	O
of	O
about	O
0.2dB/km	O
.	O
</s>
<s>
Engineers	O
overcame	O
earlier	O
difficulties	O
with	O
pulse-spreading	O
using	O
conventional	O
InGaAsP	O
semiconductor	O
lasers	O
at	O
that	O
wavelength	O
by	O
using	O
dispersion-shifted	B-Device
fibers	I-Device
designed	O
to	O
have	O
minimal	O
dispersion	O
at	O
1.55μm	O
or	O
by	O
limiting	O
the	O
laser	O
spectrum	O
to	O
a	O
single	O
longitudinal	O
mode	O
.	O
</s>
<s>
The	O
fourth	O
generation	O
of	O
fiber-optic	B-Application
communication	I-Application
systems	O
used	O
optical	O
amplification	O
to	O
reduce	O
the	O
need	O
for	O
repeaters	O
and	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
(	O
WDM	B-Architecture
)	O
to	O
increase	O
data	O
capacity	O
.	O
</s>
<s>
The	O
introduction	O
of	O
WDM	B-Architecture
was	O
the	O
start	O
of	O
optical	O
networking	O
,	O
as	O
WDM	B-Architecture
became	O
the	O
technology	O
of	O
choice	O
for	O
fiber-optic	B-Architecture
bandwidth	O
expansion	O
.	O
</s>
<s>
The	O
first	O
to	O
market	O
with	O
a	O
dense	O
WDM	B-Architecture
system	O
was	O
Ciena	O
Corp.	O
,	O
in	O
June	O
1996	O
.	O
</s>
<s>
The	O
introduction	O
of	O
optical	O
amplifiers	O
and	O
WDM	B-Architecture
caused	O
system	O
capacity	O
to	O
double	O
every	O
six	O
months	O
from	O
1992	O
until	O
a	O
bit	O
rate	O
of	O
was	O
reached	O
by	O
2001	O
.	O
</s>
<s>
The	O
focus	O
of	O
development	O
for	O
the	O
fifth	O
generation	O
of	O
fiber-optic	B-Application
communications	I-Application
is	O
on	O
extending	O
the	O
wavelength	O
range	O
over	O
which	O
a	O
WDM	B-Architecture
system	O
can	O
operate	O
.	O
</s>
<s>
In	O
the	O
late	O
1990s	O
through	O
2000	O
,	O
industry	O
promoters	O
,	O
and	O
research	O
companies	O
such	O
as	O
KMI	O
,	O
and	O
RHK	O
predicted	O
massive	O
increases	O
in	O
demand	O
for	O
communications	O
bandwidth	O
due	O
to	O
increased	O
use	O
of	O
the	O
Internet	O
,	O
and	O
commercialization	O
of	O
various	O
bandwidth-intensive	O
consumer	O
services	O
,	O
such	O
as	O
video	B-Application
on	I-Application
demand	I-Application
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
data	O
traffic	O
was	O
increasing	O
exponentially	O
,	O
at	O
a	O
faster	O
rate	O
than	O
integrated	O
circuit	O
complexity	O
had	O
increased	O
under	O
Moore	O
's	O
Law	O
.	O
</s>
<s>
Companies	O
such	O
as	O
Verizon	B-Application
and	O
AT&T	O
have	O
taken	O
advantage	O
of	O
fiber-optic	B-Application
communications	I-Application
to	O
deliver	O
a	O
variety	O
of	O
high-throughput	O
data	O
and	O
broadband	O
services	O
to	O
consumers	O
 '	O
homes	O
.	O
</s>
<s>
Modern	O
fiber-optic	B-Application
communication	I-Application
systems	O
generally	O
include	O
optical	O
transmitters	O
that	O
convert	O
electrical	O
signals	O
into	O
optical	O
signals	O
,	O
optical	B-Architecture
fiber	I-Architecture
cables	O
to	O
carry	O
the	O
signal	O
,	O
optical	O
amplifiers	O
,	O
and	O
optical	O
receivers	O
to	O
convert	O
the	O
signal	O
back	O
into	O
an	O
electrical	O
signal	O
.	O
</s>
<s>
The	O
information	O
transmitted	O
is	O
typically	O
digital	O
information	O
generated	O
by	O
computers	O
or	O
telephone	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
For	O
use	O
in	O
optical	O
communications	O
,	O
semiconductor	O
optical	O
transmitters	O
must	O
be	O
designed	O
to	O
be	O
compact	O
,	O
efficient	O
and	O
reliable	O
,	O
while	O
operating	O
in	O
an	O
optimal	O
wavelength	O
range	O
and	O
directly	O
modulated	B-Application
at	O
high	O
frequencies	O
.	O
</s>
<s>
LEDs	O
are	O
suitable	O
primarily	O
for	O
local-area-network	B-General_Concept
applications	O
with	O
bit	O
rates	O
of	O
10	O
–	O
100	O
Mbit/s	O
and	O
transmission	O
distances	O
of	O
a	O
few	O
kilometers	O
.	O
</s>
<s>
LED	O
light	O
transmission	O
is	O
inefficient	O
,	O
with	O
only	O
about	O
1%	O
of	O
input	O
power	O
,	O
or	O
about	O
100	O
microwatts	O
,	O
eventually	O
converted	O
into	O
launched	O
power	O
coupled	O
into	O
the	O
optical	B-Architecture
fiber	I-Architecture
.	O
</s>
<s>
LEDs	O
have	O
been	O
developed	O
that	O
use	O
several	O
quantum	O
wells	O
to	O
emit	O
light	O
at	O
different	O
wavelengths	O
over	O
a	O
broad	O
spectrum	O
and	O
are	O
currently	O
in	O
use	O
for	O
local-area	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
(	O
WDM	B-Architecture
)	O
applications	O
.	O
</s>
<s>
Commonly	O
used	O
classes	O
of	O
semiconductor	O
laser	O
transmitters	O
used	O
in	O
fiber	B-Architecture
optics	I-Architecture
include	O
VCSEL	O
,	O
Fabry	O
–	O
Pérot	O
and	O
distributed-feedback	O
laser	O
.	O
</s>
<s>
The	O
output	O
of	O
a	O
laser	O
is	O
relatively	O
directional	O
,	O
allowing	O
high	O
coupling	O
efficiency	O
(	O
~	O
50%	O
)	O
into	O
single-mode	B-Device
fiber	I-Device
.	O
</s>
<s>
Common	O
VCSEL	O
devices	O
also	O
couple	O
well	O
to	O
multimode	B-Device
fiber	I-Device
.	O
</s>
<s>
Furthermore	O
,	O
semiconductor	O
lasers	O
can	O
be	O
modulated	B-Application
directly	O
at	O
high	O
frequencies	O
because	O
of	O
short	O
recombination	O
time	O
.	O
</s>
<s>
Laser	O
diodes	O
are	O
often	O
directly	O
modulated	B-Application
,	O
that	O
is	O
the	O
light	O
output	O
is	O
controlled	O
by	O
a	O
current	O
applied	O
directly	O
to	O
the	O
device	O
.	O
</s>
<s>
For	O
very	O
high	O
data	O
rates	O
or	O
very	O
long	O
distance	O
links	O
,	O
a	O
laser	O
source	O
may	O
be	O
operated	O
continuous	O
wave	O
,	O
and	O
the	O
light	O
modulated	B-Application
by	O
an	O
external	O
device	O
,	O
an	O
optical	O
modulator	B-Application
,	O
such	O
as	O
an	O
electro-absorption	O
modulator	B-Application
or	O
Mach	O
–	O
Zehnder	O
interferometer	O
.	O
</s>
<s>
External	O
modulation	B-Application
increases	O
the	O
achievable	O
link	O
distance	O
by	O
eliminating	O
laser	O
chirp	B-Application
,	O
which	O
broadens	O
the	O
linewidth	O
in	O
directly	O
modulated	B-Application
lasers	O
,	O
increasing	O
the	O
chromatic	O
dispersion	O
in	O
the	O
fiber	O
.	O
</s>
<s>
For	O
very	O
high	O
bandwidth	O
efficiency	O
,	O
coherent	O
modulation	B-Application
can	O
be	O
used	O
to	O
vary	O
the	O
phase	O
of	O
the	O
light	O
in	O
addition	O
to	O
the	O
amplitude	O
,	O
enabling	O
the	O
use	O
of	O
QPSK	O
,	O
QAM	B-Device
,	O
and	O
OFDM	B-Protocol
.	O
</s>
<s>
"	O
Dual-polarization	O
quadrature	O
phase	O
shift	O
keying	O
is	O
a	O
modulation	B-Application
format	O
that	O
effectively	O
sends	O
four	O
times	O
as	O
much	O
information	O
as	O
traditional	O
optical	O
transmissions	O
of	O
the	O
same	O
speed.	O
"	O
</s>
<s>
Coherent	O
receivers	O
use	O
a	O
local	O
oscillator	O
laser	O
in	O
combination	O
with	O
a	O
pair	O
of	O
hybrid	O
couplers	O
and	O
four	O
photodetectors	O
per	O
polarization	O
,	O
followed	O
by	O
high	O
speed	O
ADCs	O
and	O
digital	O
signal	O
processing	O
to	O
recover	O
data	O
modulated	B-Application
with	O
QPSK	O
,	O
QAM	B-Device
,	O
or	O
OFDM	B-Protocol
.	O
</s>
<s>
An	O
optical	O
communication	O
system	O
transmitter	O
consists	O
of	O
a	O
digital-to-analog	O
converter	O
(	O
DAC	O
)	O
,	O
a	O
driver	O
amplifier	O
and	O
a	O
Mach	O
–	O
Zehnder	O
modulator	B-Application
.	O
</s>
<s>
The	O
deployment	O
of	O
higher	O
modulation	B-Application
formats	I-Application
(	O
>4-QAM	O
)	O
or	O
higher	O
baud	O
Rates	O
(	O
>	O
)	O
diminishes	O
the	O
system	O
performance	O
due	O
to	O
linear	O
and	O
non-linear	O
transmitter	O
effects	O
.	O
</s>
<s>
These	O
effects	O
can	O
be	O
categorized	O
as	O
linear	O
distortions	O
due	O
to	O
DAC	O
bandwidth	O
limitation	O
and	O
transmitter	O
I/Q	O
skew	O
as	O
well	O
as	O
non-linear	O
effects	O
caused	O
by	O
gain	O
saturation	O
in	O
the	O
driver	O
amplifier	O
and	O
the	O
Mach	O
–	O
Zehnder	O
modulator	B-Application
.	O
</s>
<s>
Digital	O
predistortion	O
counteracts	O
the	O
degrading	O
effects	O
and	O
enables	O
Baud	O
rates	O
up	O
to	O
and	O
modulation	B-Application
formats	I-Application
like	O
64-QAM	B-Device
and	O
128-QAM	B-Device
with	O
the	O
commercially	O
available	O
components	O
.	O
</s>
<s>
The	O
transmitter	O
digital	B-Architecture
signal	I-Architecture
processor	I-Architecture
performs	O
digital	O
predistortion	O
on	O
the	O
input	O
signals	O
using	O
the	O
inverse	O
transmitter	O
model	O
before	O
sending	O
the	O
samples	O
to	O
the	O
DAC	O
.	O
</s>
<s>
Berenguer	O
et	O
al	O
models	O
the	O
Mach	O
–	O
Zehnder	O
modulator	B-Application
as	O
an	O
independent	O
Wiener	B-Algorithm
system	I-Algorithm
and	O
the	O
DAC	O
and	O
the	O
driver	O
amplifier	O
are	O
modeled	O
by	O
a	O
truncated	O
,	O
time-invariant	O
Volterra	B-Algorithm
series	I-Algorithm
.	O
</s>
<s>
In	O
both	O
approaches	O
the	O
Volterra	B-Algorithm
series	I-Algorithm
or	O
the	O
memory	O
polynomial	O
coefficients	O
are	O
found	O
using	O
indirect-learning	O
architecture	O
.	O
</s>
<s>
Duthel	O
et	O
al	O
records	O
,	O
for	O
each	O
branch	O
of	O
the	O
Mach-Zehnder	O
modulator	B-Application
,	O
several	O
signals	O
at	O
different	O
polarity	O
and	O
phases	O
.	O
</s>
<s>
An	O
optical	B-Architecture
fiber	I-Architecture
cable	O
consists	O
of	O
a	O
core	O
,	O
cladding	B-Device
,	O
and	O
a	O
buffer	O
(	O
a	O
protective	O
outer	O
coating	O
)	O
,	O
in	O
which	O
the	O
cladding	B-Device
guides	O
the	O
light	O
along	O
the	O
core	O
by	O
using	O
the	O
method	O
of	O
total	O
internal	O
reflection	O
.	O
</s>
<s>
The	O
core	O
and	O
the	O
cladding	B-Device
(	O
which	O
has	O
a	O
lower-refractive-index	O
)	O
are	O
usually	O
made	O
of	O
high-quality	O
silica	O
glass	O
,	O
although	O
they	O
can	O
both	O
be	O
made	O
of	O
plastic	O
as	O
well	O
.	O
</s>
<s>
Connecting	O
two	O
optical	B-Architecture
fibers	I-Architecture
is	O
done	O
by	O
fusion	O
splicing	O
or	O
mechanical	B-Device
splicing	I-Device
and	O
requires	O
special	O
skills	O
and	O
interconnection	O
technology	O
due	O
to	O
the	O
microscopic	O
precision	O
required	O
to	O
align	O
the	O
fiber	O
cores	O
.	O
</s>
<s>
Two	O
main	O
types	O
of	O
optical	B-Architecture
fiber	I-Architecture
used	O
in	O
optic	O
communications	O
include	O
multi-mode	B-Device
optical	I-Device
fibers	I-Device
and	O
single-mode	B-Device
optical	I-Device
fibers	I-Device
.	O
</s>
<s>
A	O
multi-mode	B-Device
optical	I-Device
fiber	I-Device
has	O
a	O
larger	O
core	O
( ≥50	O
micrometers	O
)	O
,	O
allowing	O
less	O
precise	O
,	O
cheaper	O
transmitters	O
and	O
receivers	O
to	O
connect	O
to	O
it	O
as	O
well	O
as	O
cheaper	O
connectors	O
.	O
</s>
<s>
However	O
,	O
a	O
multi-mode	B-Device
fiber	I-Device
introduces	O
multimode	B-Device
distortion	I-Device
,	O
which	O
often	O
limits	O
the	O
bandwidth	O
and	O
length	O
of	O
the	O
link	O
.	O
</s>
<s>
Furthermore	O
,	O
because	O
of	O
its	O
higher	O
dopant	O
content	O
,	O
multi-mode	B-Device
fibers	I-Device
are	O
usually	O
expensive	O
and	O
exhibit	O
higher	O
attenuation	O
.	O
</s>
<s>
The	O
core	O
of	O
a	O
single-mode	B-Device
fiber	I-Device
is	O
smaller	O
(	O
<	O
10	O
micrometers	O
)	O
and	O
requires	O
more	O
expensive	O
components	O
and	O
interconnection	O
methods	O
,	O
but	O
allows	O
much	O
longer	O
and	O
higher-performance	O
links	O
.	O
</s>
<s>
Both	O
single	O
-	O
and	O
multi-mode	B-Device
fiber	I-Device
is	O
offered	O
in	O
different	O
grades	O
.	O
</s>
<s>
Another	O
common	O
practice	O
is	O
to	O
bundle	O
many	O
fiber	B-Architecture
optic	I-Architecture
strands	O
within	O
long-distance	O
power	O
transmission	O
cable	O
using	O
,	O
for	O
instance	O
,	O
an	O
optical	B-Device
ground	I-Device
wire	I-Device
.	O
</s>
<s>
This	O
exploits	O
power	O
transmission	O
rights	O
of	O
way	O
effectively	O
,	O
ensures	O
a	O
power	O
company	O
can	O
own	O
and	O
control	O
the	O
fiber	O
required	O
to	O
monitor	O
its	O
own	O
devices	O
and	O
lines	O
,	O
is	O
effectively	O
immune	O
to	O
tampering	O
,	O
and	O
simplifies	O
the	O
deployment	O
of	O
smart	B-Architecture
grid	I-Architecture
technology	O
.	O
</s>
<s>
The	O
transmission	O
distance	O
of	O
a	O
fiber-optic	B-Application
communication	I-Application
system	O
has	O
traditionally	O
been	O
limited	O
by	O
fiber	O
attenuation	O
and	O
by	O
fiber	O
distortion	O
.	O
</s>
<s>
Because	O
of	O
the	O
high	O
complexity	O
with	O
modern	O
wavelength-division	B-Architecture
multiplexed	I-Architecture
signals	O
,	O
including	O
the	O
fact	O
that	O
they	O
had	O
to	O
be	O
installed	O
about	O
once	O
every	O
,	O
the	O
cost	O
of	O
these	O
repeaters	O
is	O
very	O
high	O
.	O
</s>
<s>
These	O
are	O
made	O
by	O
doping	B-Algorithm
a	O
length	O
of	O
fiber	O
with	O
the	O
rare-earth	O
mineral	O
erbium	O
and	O
laser	O
pumping	O
it	O
with	O
light	O
with	O
a	O
shorter	O
wavelength	O
than	O
the	O
communications	O
signal	O
(	O
typically	O
980nm	O
)	O
.	O
</s>
<s>
First	O
,	O
an	O
optical	O
amplifier	O
can	O
amplify	O
a	O
very	O
wide	O
band	O
at	O
once	O
which	O
can	O
include	O
hundreds	O
of	O
multiplexed	B-Architecture
channels	O
,	O
eliminating	O
the	O
need	O
to	O
demultiplex	O
signals	O
at	O
each	O
amplifier	O
.	O
</s>
<s>
Second	O
,	O
optical	O
amplifiers	O
operate	O
independently	O
of	O
the	O
data	O
rate	O
and	O
modulation	B-Application
format	O
,	O
enabling	O
multiple	O
data	O
rates	O
and	O
modulation	B-Application
formats	I-Application
to	O
co-exist	O
and	O
enabling	O
upgrading	O
of	O
the	O
data	O
rate	O
of	O
a	O
system	O
without	O
having	O
to	O
replace	O
all	O
of	O
the	O
repeaters	O
.	O
</s>
<s>
Wavelength-division	B-Architecture
multiplexing	I-Architecture
(	O
WDM	B-Architecture
)	O
is	O
the	O
technique	O
of	O
transmitting	O
multiple	O
channels	O
of	O
information	O
through	O
a	O
single	O
optical	B-Architecture
fiber	I-Architecture
by	O
sending	O
multiple	O
light	O
beams	O
of	O
different	O
wavelengths	O
through	O
the	O
fiber	O
,	O
each	O
modulated	B-Application
with	O
a	O
separate	O
information	O
channel	O
.	O
</s>
<s>
This	O
allows	O
the	O
available	O
capacity	O
of	O
optical	B-Architecture
fibers	I-Architecture
to	O
be	O
multiplied	O
.	O
</s>
<s>
Arrayed	B-Protocol
waveguide	I-Protocol
gratings	I-Protocol
are	O
commonly	O
used	O
for	O
multiplexing	O
and	O
demultiplexing	O
in	O
WDM	B-Architecture
.	O
</s>
<s>
Using	O
WDM	B-Architecture
technology	O
now	O
commercially	O
available	O
,	O
the	O
bandwidth	O
of	O
a	O
fiber	O
can	O
be	O
divided	O
into	O
as	O
many	O
as	O
160	O
channels	O
to	O
support	O
a	O
combined	O
bit	O
rate	O
in	O
the	O
range	O
of	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
common	O
multi-mode	B-Device
fiber	I-Device
with	O
bandwidth	O
–	O
distance	O
product	O
of	O
500MHz·km	O
could	O
carry	O
a	O
500MHz	O
signal	O
for	O
1km	O
or	O
a	O
1000MHz	O
signal	O
for	O
0.5km	O
.	O
</s>
<s>
Using	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
,	O
each	O
fiber	O
can	O
carry	O
many	O
independent	O
channels	O
,	O
each	O
using	O
a	O
different	O
wavelength	O
of	O
light	O
.	O
</s>
<s>
The	O
net	O
data	O
rate	O
(	O
data	O
rate	O
without	O
overhead	O
bytes	O
)	O
per	O
fiber	O
is	O
the	O
per-channel	O
data	O
rate	O
reduced	O
by	O
the	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
overhead	O
,	O
multiplied	O
by	O
the	O
number	O
of	O
channels	O
(	O
usually	O
up	O
to	O
eighty	O
in	O
commercial	O
dense	O
WDM	B-Architecture
systems	O
)	O
.	O
</s>
<s>
The	O
following	O
summarizes	O
research	O
using	O
specialized	O
cables	O
that	O
allow	O
spatial	O
multiplexing	O
to	O
occur	O
,	O
use	O
specialized	O
tri-mode	O
fiber	O
cables	O
or	O
similar	O
specialized	O
fiber	B-Architecture
optic	I-Architecture
cables	O
.	O
</s>
<s>
Research	O
conducted	O
by	O
the	O
RMIT	O
University	O
,	O
Melbourne	O
,	O
Australia	O
,	O
have	O
developed	O
a	O
nanophotonic	O
device	O
that	O
has	O
achieved	O
a	O
100-fold	O
increase	O
in	O
current	O
attainable	O
fiber	B-Architecture
optic	I-Architecture
speeds	O
by	O
using	O
a	O
twisted-light	O
technique	O
.	O
</s>
<s>
The	O
nanophotonic	O
device	O
uses	O
ultra	O
thin	O
topological	O
nanosheets	O
to	O
measure	O
a	O
fraction	O
of	O
a	O
millimeter	O
of	O
twisted	O
light	O
,	O
the	O
nano-electronic	O
device	O
is	O
embedded	O
within	O
a	O
connector	O
smaller	O
than	O
the	O
size	O
of	O
a	O
USB	O
connector	O
,	O
it	O
fits	O
easily	O
at	O
the	O
end	O
of	O
an	O
optical	B-Architecture
fiber	I-Architecture
cable	O
.	O
</s>
<s>
For	O
modern	O
glass	O
optical	B-Architecture
fiber	I-Architecture
,	O
the	O
maximum	O
transmission	O
distance	O
is	O
limited	O
not	O
by	O
direct	O
material	O
absorption	O
but	O
by	O
several	O
types	O
of	O
dispersion	O
,	O
or	O
spreading	O
of	O
optical	O
pulses	O
as	O
they	O
travel	O
along	O
the	O
fiber	O
.	O
</s>
<s>
Dispersion	O
in	O
optical	B-Architecture
fibers	I-Architecture
is	O
caused	O
by	O
a	O
variety	O
of	O
factors	O
.	O
</s>
<s>
Intermodal	B-Device
dispersion	I-Device
,	O
caused	O
by	O
the	O
different	O
axial	O
speeds	O
of	O
different	O
transverse	O
modes	O
,	O
limits	O
the	O
performance	O
of	O
multi-mode	B-Device
fiber	I-Device
.	O
</s>
<s>
Because	O
single-mode	B-Device
fiber	I-Device
supports	O
only	O
one	O
transverse	O
mode	O
,	O
intermodal	B-Device
dispersion	I-Device
is	O
eliminated	O
.	O
</s>
<s>
In	O
single-mode	B-Device
fiber	I-Device
performance	O
is	O
primarily	O
limited	O
by	O
chromatic	O
dispersion	O
(	O
also	O
called	O
group	O
velocity	O
dispersion	O
)	O
,	O
which	O
occurs	O
because	O
the	O
index	O
of	O
the	O
glass	O
varies	O
slightly	O
depending	O
on	O
the	O
wavelength	O
of	O
the	O
light	O
,	O
and	O
light	O
from	O
real	O
optical	O
transmitters	O
necessarily	O
has	O
nonzero	O
spectral	O
width	O
(	O
due	O
to	O
modulation	B-Application
)	O
.	O
</s>
<s>
Polarization	O
mode	O
dispersion	O
,	O
another	O
source	O
of	O
limitation	O
,	O
occurs	O
because	O
although	O
the	O
single-mode	B-Device
fiber	I-Device
can	O
sustain	O
only	O
one	O
transverse	O
mode	O
,	O
it	O
can	O
carry	O
this	O
mode	O
with	O
two	O
different	O
polarizations	O
,	O
and	O
slight	O
imperfections	O
or	O
distortions	O
in	O
a	O
fiber	O
can	O
alter	O
the	O
propagation	O
velocities	O
for	O
the	O
two	O
polarizations	O
.	O
</s>
<s>
This	O
phenomenon	O
is	O
called	O
fiber	O
birefringence	O
and	O
can	O
be	O
counteracted	O
by	O
polarization-maintaining	B-Device
optical	I-Device
fiber	I-Device
.	O
</s>
<s>
Although	O
material	O
absorption	O
for	O
pure	O
silica	O
is	O
only	O
around	O
0.03dB/km	O
(	O
modern	O
fiber	O
has	O
attenuation	O
around	O
0.3dB/km	O
)	O
,	O
impurities	O
in	O
the	O
original	O
optical	B-Architecture
fibers	I-Architecture
caused	O
attenuation	O
of	O
about	O
1000dB/km	O
.	O
</s>
<s>
When	O
a	O
communications	O
link	O
must	O
span	O
a	O
larger	O
distance	O
than	O
existing	O
fiber-optic	B-Architecture
technology	I-Architecture
is	O
capable	O
of	O
,	O
the	O
signal	O
must	O
be	O
regenerated	O
at	O
intermediate	O
points	O
in	O
the	O
link	O
by	O
optical	B-Protocol
communications	I-Protocol
repeaters	I-Protocol
.	O
</s>
<s>
Although	O
fiber-optic	B-Architecture
systems	O
excel	O
in	O
high-bandwidth	O
applications	O
,	O
optical	B-Architecture
fiber	I-Architecture
has	O
been	O
slow	O
to	O
achieve	O
its	O
goal	O
of	O
fiber	B-Architecture
to	I-Architecture
the	I-Architecture
premises	I-Architecture
or	O
to	O
solve	O
the	O
last	B-Protocol
mile	I-Protocol
problem	O
.	O
</s>
<s>
However	O
,	O
FTTH	B-Architecture
deployment	O
has	O
increased	O
significantly	O
over	O
the	O
last	O
decade	O
and	O
is	O
projected	O
to	O
serve	O
millions	O
more	O
subscribers	O
in	O
the	O
near	O
future	O
.	O
</s>
<s>
In	O
Japan	O
,	O
for	O
instance	O
EPON	B-Architecture
has	O
largely	O
replaced	O
DSL	O
as	O
a	O
broadband	O
Internet	O
source	O
.	O
</s>
<s>
South	O
Korea	O
's	O
KT	O
also	O
provides	O
a	O
service	O
called	O
FTTH	B-Architecture
(	O
Fiber	B-Architecture
To	I-Architecture
The	I-Architecture
Home	I-Architecture
)	O
,	O
which	O
provides	O
fiber-optic	B-Architecture
connections	O
to	O
the	O
subscriber	O
's	O
home	O
.	O
</s>
<s>
The	O
largest	O
FTTH	B-Architecture
deployments	O
are	O
in	O
Japan	O
,	O
South	O
Korea	O
,	O
and	O
China	O
.	O
</s>
<s>
In	O
the	O
US	O
,	O
Verizon	B-Application
Communications	O
provides	O
a	O
FTTH	B-Architecture
service	O
called	O
FiOS	B-Protocol
to	O
select	O
high-ARPU	O
(	O
Average	O
Revenue	O
Per	O
User	O
)	O
markets	O
within	O
its	O
existing	O
territory	O
.	O
</s>
<s>
The	O
other	O
major	O
surviving	O
ILEC	O
(	O
or	O
Incumbent	O
Local	O
Exchange	O
Carrier	O
)	O
,	O
AT&T	O
,	O
uses	O
a	O
FTTN	B-Architecture
(	O
Fiber	B-Architecture
To	I-Architecture
The	I-Architecture
Node	I-Architecture
)	O
service	O
called	O
U-verse	O
with	O
twisted-pair	O
to	O
the	O
home	O
.	O
</s>
<s>
Their	O
MSO	O
competitors	O
employ	O
FTTN	B-Architecture
with	O
coax	O
using	O
HFC	B-Architecture
.	O
</s>
<s>
The	O
globally	O
dominant	O
access	O
network	O
technology	O
is	O
EPON	B-Architecture
(	O
Ethernet	O
Passive	B-Architecture
Optical	I-Architecture
Network	I-Architecture
)	O
.	O
</s>
<s>
In	O
Europe	O
,	O
and	O
among	O
telcos	O
in	O
the	O
United	O
States	O
,	O
BPON	O
(	O
ATM-based	O
Broadband	B-Architecture
PON	I-Architecture
)	O
and	O
GPON	B-Architecture
(	O
Gigabit	B-Architecture
PON	I-Architecture
)	O
had	O
roots	O
in	O
the	O
FSAN	O
(	O
Full	O
Service	O
Access	O
Network	O
)	O
and	O
ITU-T	O
standards	O
organizations	O
under	O
their	O
control	O
.	O
</s>
<s>
The	O
choice	O
between	O
optical	B-Architecture
fiber	I-Architecture
and	O
electrical	O
(	O
or	O
copper	O
)	O
transmission	O
for	O
a	O
particular	O
system	O
is	O
made	O
based	O
on	O
a	O
number	O
of	O
trade-offs	O
.	O
</s>
<s>
Optical	B-Architecture
fiber	I-Architecture
is	O
generally	O
chosen	O
for	O
systems	O
requiring	O
higher	O
bandwidth	O
or	O
spanning	O
longer	O
distances	O
than	O
electrical	O
cabling	O
can	O
accommodate	O
.	O
</s>
<s>
The	O
main	O
benefits	O
of	O
fiber	O
are	O
its	O
exceptionally	O
low	O
loss	O
(	O
allowing	O
long	O
distances	O
between	O
amplifiers/repeaters	O
)	O
,	O
its	O
absence	O
of	O
ground	O
currents	O
and	O
other	O
parasite	O
signal	O
and	O
power	O
issues	O
common	O
to	O
long	O
parallel	O
electric	O
conductor	O
runs	O
(	O
due	O
to	O
its	O
reliance	O
on	O
light	O
rather	O
than	O
electricity	O
for	O
transmission	O
,	O
and	O
the	O
dielectric	O
nature	O
of	O
fiber	B-Architecture
optic	I-Architecture
)	O
,	O
and	O
its	O
inherently	O
high	O
data-carrying	O
capacity	O
.	O
</s>
<s>
Single-mode	B-Device
fiber	I-Device
cables	O
are	O
commonly	O
available	O
in	O
lengths	O
,	O
minimizing	O
the	O
number	O
of	O
splices	O
required	O
over	O
a	O
long	O
cable	O
run	O
.	O
</s>
<s>
Multi-mode	B-Device
fiber	I-Device
is	O
available	O
in	O
lengths	O
up	O
to	O
4km	O
,	O
although	O
industrial	O
standards	O
only	O
mandate	O
2km	O
unbroken	O
runs	O
.	O
</s>
<s>
Optical	B-Architecture
fibers	I-Architecture
are	O
more	O
difficult	O
and	O
expensive	O
to	O
splice	O
than	O
electrical	O
conductors	O
.	O
</s>
<s>
And	O
at	O
higher	O
powers	O
,	O
optical	B-Architecture
fibers	I-Architecture
are	O
susceptible	O
to	O
fiber	O
fuse	O
,	O
resulting	O
in	O
catastrophic	O
destruction	O
of	O
the	O
fiber	O
core	O
and	O
damage	O
to	O
transmission	O
components	O
.	O
</s>
<s>
Because	O
of	O
these	O
benefits	O
of	O
electrical	O
transmission	O
,	O
optical	O
communication	O
is	O
not	O
common	O
in	O
short	O
box-to-box	O
,	O
backplane	B-Architecture
,	O
or	O
chip-to-chip	O
applications	O
;	O
however	O
,	O
optical	O
systems	O
on	O
those	O
scales	O
have	O
been	O
demonstrated	O
in	O
the	O
laboratory	O
.	O
</s>
<s>
Optical	B-Architecture
fiber	I-Architecture
cables	O
can	O
be	O
installed	O
in	O
buildings	O
with	O
the	O
same	O
equipment	O
that	O
is	O
used	O
to	O
install	O
copper	O
and	O
coaxial	O
cables	O
,	O
with	O
some	O
modifications	O
due	O
to	O
the	O
small	O
size	O
and	O
limited	O
pull	O
tension	O
and	O
bend	O
radius	O
of	O
optical	O
cables	O
.	O
</s>
<s>
In	O
order	O
for	O
various	O
manufacturers	O
to	O
be	O
able	O
to	O
develop	O
components	O
that	O
function	O
compatibly	O
in	O
fiber	B-Application
optic	I-Application
communication	I-Application
systems	O
,	O
a	O
number	O
of	O
standards	O
have	O
been	O
developed	O
.	O
</s>
<s>
TOSLINK	B-Protocol
is	O
the	O
most	O
common	O
format	O
for	O
digital	O
audio	O
cable	O
using	O
plastic	B-Application
optical	I-Application
fiber	I-Application
to	O
connect	O
digital	O
sources	O
to	O
digital	O
receivers	O
.	O
</s>
